Apparatus for producing corrugated tube

ABSTRACT

An apparatus for producing a corrugated tube contains a roller assembly composed of plural rollers having plural convexed rings, respective convexed rings forming a helix as a whole so as to substantially form a single rotating shaft body, and a synthetic resin supplier for supplying a strip of synthetic resin onto the roller assembly. The pitches between the convexed rings, and the diameters and the widths of the convexed rings are so constructed as to be gradually decreased in accordance with the contraction percentage of the synthetic resin. The apparatus is provided with a follower ring on a roller valley portion between the convexed rings, the follower ring being rotatable in response to circumferential speed of the apex of the convexed ring.

TECHNICAL FIELD

This invention relates to an apparatus for producing a corrugated tubeof synthetic resin. More particularly, it relates to an apparatus forproducing a corrugated tube in which the cross-sectional shape of theouter circumferential surface of the tube is corrugated and whichexhibits good flattening strength and especially high molding stabilityand high durability.

BACKGROUND TECHNOLOGY

Various proposals have hitherto been made for an apparatus for producinga corrugated tube of synthetic resin so that the outer circumferentialsurface of the tube has a corrugated cross-sectional profile. Forexample, there is known an apparatus comprising a roller assembly madeup of plural rollers forming an imaginary cylinder, the rollers beingeach provided with plural convexed rings of the same size, and beingrotated about their own axes at equal speed. The rollers are arrangedparallel to one another so that the axes of the rollers are on thecircumference surface of the imaginary cylinder and. The axialdirections of the rollers are circumferentially inclined and parallel toone another. A die is provided for continuously supplying a strip ofsynthetic resin in a molten state from the lateral side onto theperiphery of the roller assembly. A driving motor continuously rotatesthe rollers at equal speed. There is also known an apparatus (JapanesePatent Publication No. 62-60261 (1987)) which further comprises apressure roller profiled for meshing with the profile of the outer wallsurface of each roller for fixing the shape of the strip of syntheticresin, means for cooling the synthetic resin as it is shaped to atubular profile as it is moved along the outer periphery of theimaginary cylinder, and forming means for forming a smoothcylindrical-shaped synthetic resin on the inner side of the corrugatedtube for providing a double-walled structure.

However, with the above-described known apparatuses, as the strip ofsynthetic resin is wound spirally and advanced along the axis of theroller assembly while being cooled and cured, the strip itself contractsin size due to cooling. Since the convexed rings formed on each rollerare of the same size and arrayed at an equal interval on the roller, thesynthetic resin strip is actually being cured under conditions in whichthe strip is pulled outwardly along the shape of each convexed ring. Theresult is that, as the strip of synthetic resin progresses axially alongthe roller assembly, the strip tends to be detached from the valleyportions between the convexed rings to detract from molding stability,accuracy and durability of the ultimate corrugated tube.

Besides, with the above-described known apparatus, although each rollerof the roller assembly is rotated at equal speed, a difference incircumferential speed of about 10 to 45% is produced between (a)circumferential speed at the apex of the convexed portion and (b) speedin the recessed interstices between neighboring convexed portions ofeach roller, so that longitudinal creases (internal strain) are producedbetween the convexed portions and the recessed valley portions of thewall surfaces of the corrugated tube. As a result, the corrugated tubeproduced is lowered in durability, such as by reduced impact strength orpressure resistance.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide an apparatus forproducing a corrugated tube which is capable of preventing a strip ofsynthetic resin from being detached from the rollers due to contractionin size and which is superior in durability, such as by increased impactstrength or pressure resistance.

It is another object of the present invention to provide an apparatusfor producing a corrugated tube which is superior in durability, such asby increased impact strength or pressure resistance and in whichlongitudinal creases (internal strain) between the convexed portions andthe valley portions thereof are not formed.

According to the present invention, there is provided an apparatus forproducing a corrugated tube comprising a roller assembly composed of aplurality of rollers, each being rotatable at equal speed. The rollerassembly substantially forms a single rotating axial body so that therollers means are arrayed on a cylindrical surface of an imaginarycylinder. Roller supporting means are mounted axially opposite theroller assembly for rotatably supporting the rollers. Means are providedfor continuously supplying a synthetic resin strip to the outercircumferential surface of the roller assembly from a lateral sidethereof. Driving means are provided for rotating the rollers at equalspeed. Each axis of the rollers is inclined in a circumferentialdirection of the roller assembly. Plural convexed rings are providedalong the axial direction of each of the rollers arrayed so that theconvexed rings form a helix in their entirety. The pitch between theconvexed rings and diameters and widths of the convexed rings on each ofthe rollers is gradually reduced in accordance with contractionpercentage of said synthetic resin strip at least within a section ofsaid rollers means in which the synthetic resin strip which is spirallywound and advanced along the axis of the roller assembly is contracted(referred to hereinafter as a first apparatus).

According to the present invention, there is further provided anapparatus for producing a corrugated tube comprising a roller assemblycomposed of plural rollers, each being rotatable at equal speed. Theroller assembly substantially forms a single rotating axial body so thatthe rollers are arrayed on a cylindrical surface of an imaginarycylinder. Roller supporting means are mounted axially opposite theroller assembly for rotatably supporting said rollers. Means areprovided for continuously supplying a synthetic resin strip to the outercircumferential surface of the roller assembly from a lateral sidethereof. A Driving means are provided for rotating the rollers at equalspeed. Each axis of the rollers is inclined in a circumferentialdirection of the roller assembly. Plural convexed rings are providedalong the axial direction of each of the rollers arrayed so that theconvexed rings form a helix in their entirety. A follower ring isprovided at a recessed valley portion between neighboring convexedrings. The follower ring is rotatable responsive to the circumferentialspeed at an apex of the convexed ring (referred to hereinafter as asecond apparatus).

According to the present invention, there is also provided an apparatusfor producing a corrugated tube which corresponds to the first apparatusfitted with the aforementioned follower ring (referred to hereinafter asthe third apparatus).

In each of the apparatuses for producing the corrugated tube, a pressureroller having a convexed shape mating with the convexed shape of therollers may be provided for fixing the shape of the strip winding in ahelix.

In each of the apparatuses for producing the corrugated tube, thecross-sectional shape of the circumferencial surface of the tubecoincident with the convexed shape of the plural rollers is corrugated.An inner cylindrical synthetic resin forming means may be provided forforming a smooth cylindrical synthetic resin on the inner side of thecorrugated tube wall surface, that is on the inner peripheral surface ofthe imaginary cylinder, for providing a dual wall for the corrugatedtube. Besides, cooling means may be provided within each roller andwithin the inner cylindrical synthetic resin forming means for coolingthe strip of synthetic resin as it is wound.

In each of the above-described apparatuses for producing the corrugatedtube, it suffices if each axis of the rollers is circumferentiallyinclined relative to the axis of the roller assembly at an angle ofinclination which allows the strip of synthetic resin to be movedcontinuously in the axially extending direction of the roll assembly forcontinuously producing the spiral-shaped tube. In addition, each axis ofthe rollers may be circumferentially inclined relative to the axis ofthe roller assembly, while at the same time each axis of the rollers maybe equally inclined toward the central axis of the roller assembly sothat the diameter of the imaginary cylinder is decreased gradually inthe axially extending direction. By inclining the rollers so that theimaginary cylinder is decreased gradually in diameter, it becomespossible to prevent the molding stability and accuracy from beinglowered due to the contraction caused by cooling of the strip ofsynthetic resin. At least one of the roller supporting means may beprovided with inclination angle adjustment means for adjusting the angleof inclination of the rollers.

The synthetic resin employed for each of the above-described apparatusesfor producing the corrugated tube is preferably thermoplastic resin,such as polyethylene, polypropylene or polyvinyl chloride. Incidentally,the strip of synthetic resin may be a flat strip when supplied from thesupplying means or may be supplied in a corrugated form mating with thecorrugations of the rollers.

In the above-described first and third apparatuses, the pitch betweenthe convexed rings, and the diameters and the widths of the convexedrings may be decreased gradually in accordance with the contractionpercentage of the strip of synthetic resin. Thus it becomes possible toprevent the strip from becoming detached from the recessed valleyportions between the neighboring convexed rings of the rollers due tocontraction of the synthetic resin, which would otherwise occur if theconvexed rings on the rollers were of the same size and arrayed at equalintervals. As a result the corrugated tube produced is improved byincreased molding stability, accuracy and durability. The rate ofdecrease of the pitch between neighboring convexed rings and thediameters and widths of the convexed rings may be selected depending onthe type of synthetic resin employed. For example, if a general-purposepolyethylene is employed as a substrate for the strip of syntheticresin, it has a molding shrinkage percentage of approximately 2% and iscontracted by about 10% after being wound, depending on the tensionapplied when the strip is being wound, so that the rate of decrease maybe set depending on the contraction percentage. While the rate ofdecrease varies depending on the synthetic resins employed, when thepitch between the convexed rings and the diameter and the width of theconvexed ring at the synthetic resin strip supplying position is equalto X, and the minimum value is equal to Y, the ratio Y/X is given by0.8<Y/X<1 and especially 0.9<Y/X<1.

The section or range within which the pitch between the convexed ringsand the diameter and the width of the convexed rings are to be reducedneeds only to be at least such a section within which contraction due tocuring of the strip of synthetic resin takes place as the strip isspirally wound along the length of the rollers. For those convexed ringswhich come into use as shape retention means for the spirally wound tubeafter the termination of shrinkage or curing of the strip of syntheticresin, it is not necessary to change the above values, so that theconvexed rings of the same shape may be arranged with the same uniformpitch.

The follower ring which is employed in the above-described second andthird apparatuses and which may be rotated responsive to thecircumferential speed of the apex portions of the convexed rings,prevents longitudinal creases (internally as strains) from occurringbetween the convexed portions and valley portions of the wall surface ofthe corrugated tube due to the rollers making up the roller assemblybeing rotated at an equal speed to produce a difference in thecircumferential speed on the order of about 10 to 45% between the apexpoint of the convexed ring and the valley portions between the rings forimproving durability such as increased impact strength or pressureresistance. There is no particular limitation to the follower ring if itis capable of being rotated at the circumferential speed of the strip ofsynthetic resin at the apex point of the convexed ring so that when thestrip travels in contact with the convexed portions of the rollers andthe valley portions, the strip may be rotated at an equalcircumferential speed at the apex point and at the valley portionwithout regard to the rotational speed of the shafts of the rollers.Thus, the follower rings may be formed as rings freely rotatably mountedon the valley portions via bearings or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side elevational view showing an apparatus forproducing a corrugated tube according to an embodiment of the presentinvention.

FIG. 2 is an enlarged view showing a roller having convexed rings shownin FIG. 1.

FIG. 3 is a schematic back-side view showing the apparatus shown in FIG.1.

FIG. 4 is a front view showing a supporting plate in the apparatus,shown in FIG. 1, with only an arrangement of the rollers beingschematically shown for ease of understanding.

FIG. 5 is a schematic view showing perspectively the inside of a part ofa roller assembly in the apparatus shown in FIG. 1.

FIG. 6 is a schematic side elevational view showing an apparatus forproducing a corrugated tube according to a modified embodiment of thepresent invention.

FIG. 7 is an enlarged schematic partial cross-sectional view of a rollershown in FIG. 6.

FIG. 8 is a schematic cross-sectional view taken along line 8--8 in FIG.7.

FIG. 9 is an enlarged schematic view of a roller having convexed ringsand follower rings provided in valley portions between neighboringconvexed rings, with the pitch between neighboring convexed rings andthe diameters and the widths of the convexed rings being decreasedgradually along the axial direction.

BEST MODE FOR PRACTICING THE INVENTION

The present invention will be explained hereinbelow by referring to thedrawings showing a preferred but non-limitative form of the invention.

In FIG. 1, 1 is an apparatus for producing a corrugated tube comprisedof a supporting frame 2, a disk-shaped supporting plate 4 opposed to thesupporting frame 2, nine rollers 3a to 3i making up a roller assembly 3,which is arranged along an imaginary circle between the supporting frame2 and the supporting plate 4 and which substantially forms a singlerotating shaft body, a motor 10 for rotating the rollers 3a to 3i atequal speed and a die 8 for supplying a strip of synthetic resin A tothe roller assembly 3.

A first half portion of each of the rollers 3a to 3i towards thesupporting frame 2 is provided with seven annular convexed rings 9 onits outer peripheral surface, as shown in FIG. 2. The pitches P₁ to P₆,the diameters Q₁ to Q₇ and the widths R₁ to R₇ of the convexed rings 9are selected so as to be gradually decreased from the supporting frame 2towards the supporting plate 4 in FIG. 1 so that the ratios P₆ /P₁, Q₇/Q₁ and R₇ /R₁ amount to 0.9. A second half portion of each of therollers 3a to 3i towards the supporting plate 4 is a smooth cylinder.Since the changes in the pitches, the diameters and the widths of theconvexed rings shown in FIG. 1 are only small in contrast to the size ofthe entire apparatus, they are shown with substantially the same sizewhich is different from actual sizes.

The supporting plate 4 is mounted on the distal end of a hollow baseshaft 5 passed through the inside of the supporting frame 2. Shaftportions 6, 7 of each roller 3a to 3i is rotatably supported inbearings. Each shaft portion 6 passed through the supporting frame 2 isadapted to rotate at equal speed under the driving force of the motor 10by interconnecting, by means of endless chains 15, 16, each sprocketloaded on each of the rollers 3a to 3i, as shown in FIG. 3, a sprocket12 loosely mounted on a shaft 11 set on the supporting frame 2, and asprocket 14 mounted on a speed-reducing unit 13 connected to the motor10. A sprocket 17 is employed for tension adjustment of the endlesschain 15. By such arrangement, when the sprocket 14 of thespeed-reducing unit 13 is rotated by the motor 10, the sprockets of therollers 3a to 3g are rotated by the movement of the endless chain 15and, with the rotation of the endless chain 15, the endless chain 16 ismoved to rotate the sprockets of the rollers 3a to 3i to cause therotation of the rollers 3a to 3i at equal speed.

Each shaft portion 7 passed through the supporting plate 4 is rotatablysupported by being passed through each elliptically shaped bearing 7a(see FIG. 4) capable of adjusting the angle of inclination of the rollershaft. If the shaft portions 7 are supported at the radially outermostpositions of the imaginary cylinder 18 as shown in FIG. 4, the rollers3a to 3i of the roller assembly 3 are supported in their entirety inparallel with one another relative to the axial direction x of the baseshaft 5 with an inclination angle of x-y. If each shaft portion 7 issupported by the elliptically shaped bearings 7a at the radiallyinnermost positions of the imaginary cylinder 18, the roller assembly 3may be supported so that the diameter of the imaginary cylinder 18 shownin FIG. 4 is gradually decreased in the direction proceeding from thesupporting frame 2 towards the supporting plate 4. By supporting theshaft portions 7 so that the diameter of the imaginary cylinder 18 isdecreased gradually, it becomes possible to prevent the spirally woundstrip of synthetic resin A from being detached from the valleys of therollers 3a to 3i as a result of contraction due to cooling of the stripA.

In FIG. 1, 19 is a pressure roller having a corrugated profilecorresponding to the seven convexed rings 9 on the rollers 3a to 3i forpressing the strip of synthetic resin A discharged from the die 8 ontothe peripheral surface of the roller assembly 3 for fixing the shape ofthe strip A. The pressure roller 19 is rotatably mounted externally ofthe roller assembly 3 constituting the imaginary cylinder. 20 is a guideroller for guiding the strip A as it is discharged onto the rollerassembly 3. The guide roller 20 is mounted externally of and parallel tothe roller assembly 3.

In FIG. 5, 21 is a die for discharging a strip of synthetic resin Bwhich forms a smooth cylindrical surface which is to become the innerwall surface of the corrugated tube. The die 21 is connected to amandrel 22 mounted inside of the base shaft 5, and is mounted at a midportion within the inside of the roller assembly 3 so that the strip ofsynthetic resin B is supplied to the smooth portions of the rollers 3ato 3i.

Within the inside of the rollers 3a to 3i, there is provided a knownunit, not shown, for circulation of cooling water for cooling the stripof synthetic resin A which is wound spirally as it is discharged fromthe die 8.

The process for producing a corrugated tube using the above-describedapparatus 1 is hereinafter explained.

Referring to FIG. 1, the strip of synthetic resin A discharged from thedie 8 is continuously discharged via the guide roller 20 onto the outerperiphery of the rollers 3a to 3i, while at the same time the motor 10is driven in rotation for rotating the rollers 3a to 3i at equal speedmatched to the rate of supply of the strip A. In this manner, the stripA is supplied sequentially to points m4, m3, m2 and m1 on the outerperiphery of the roller assembly 3 shown in FIG. 1. After one round ofthe strip A is wound around the outer periphery of the roller assembly3, that portion of the strip A which has made a complete round ispartially overlapped with a portion of the strip A to be subsequentlysupplied in the similar manner, and is advanced by virtue of theinclination of the rollers 3a to 3i to form a complete turn of thehelix. The strip A is moved past the pressure roller 19 in the vicinityof the roller 3i so that the strip A is fixed in a corrugated shape bythe cross-sectional shape of the roller assembly 3 in cooperation withthe contour of the pressure roller 19. When the strip A reaches the lastconvexed ring 9 provided on each of the rollers 3a to 3i, a strip ofsynthetic resin B is discharged from the die 21 shown in FIG. 5 and issupplied to the smooth portions of the rollers 3a to 3i. The strip B isadvanced gradually and caused to be continuously bonded to the innersurface of the strip A wound. The strip B is in a helical form to definea cylindrical inner surface of the corrugated tube. The strip B isadvanced in the direction of the supporting plate 4 by taking advantageof the inclination of the shafts of the rollers 3a to 3i, therebycontinuously producing a corrugated tube having a double-wall structurecomprising a corrugated outer wall and a smooth cylindrical inner wall.

The wave-shaped outer wall of the corrugated tube continuously producedin this manner is cured and contracted by being cooled by cooling means,not shown, provided within the inside of the rollers 3a to 3i. Thepitches, the diameters and the widths of the convexed rings 9 providedon the rollers 3a to 3i are reduced in the direction proceeding from thesupporting frame 2 towards the supporting plate 4 at rates related tothe contraction percentages of the various portions of the strip A inthe apparatus 1 as shown in FIG. 2. As a result, the wave-shaped outerwall of the corrugated tube is continuously formed without the strip Adisengaging from the valley portion between the last convexed ring 9 andthe second to last convexed ring 9 of each roller 3a to 3i, withoutbeing effected by the contraction of the strip A caused by cooling.Consequently, the corrugated tube produced is improved by increasedmolding shape stability and accuracy while being superior in durability.The molding shape stability and accuracy may be further improved byinclining the axial direction of each of the rollers 3a to 3i so as togradually reduce the diameter of the roller assembly 3 in addition tothe adjustment of the pitches, diameters and widths of the convexedrings.

Referring to FIG. 6, 60 is an apparatus for producing a corrugated tubeaccording to a modified embodiment of the present invention. Theapparatus 60 includes nine rollers 63a to 63i in place of the rollerassembly of the apparatus 1 shown in FIG. 1, wherein the rollers 63a to63i are each provided with eight convexed rings 62 at the same pitch.Incidentally, the die 2 for forming the smooth inner surface shown inFIG. 5 is not installed. The embodiment shown in FIG. 6 is otherwise thesame as that shown in FIG. 1, so that various parts or components of thepresent embodiment are denoted by the same reference numerals and thecorresponding description is not made.

FIG. 7 shows the rollers 63a to 63i shown in FIG. 6 by a partiallyenlarged cross-sectional view, and FIG. 8 is a cross-section taken alongline 8--8 in FIG. 7.

Each of the rollers 63a to 63i is formed with convexed rings 62 of thesame shape and size. On a roll substrate 64 in each valley portionbetween neighboring convexed rings 62, each follower ring 61a isrotatably mounted via dry bearings 61b and a positioning fixture 61cfixed by a fitting 65 without regard to the rotation of the rollsubstrate 64. Cooling means, not shown, for circulating cooling water ina manner well-known per se, is provided internally of each of therollers 63a to 63i.

The method for producing a corrugated tube by the above-describedapparatus 60 is hereinafter explained.

Referring to FIG. 6, the strip of synthetic resin A discharged from thedie 8 is continuously discharged onto the outer periphery of the rollers63a to 63i via the guide roller 20. Simultaneously, the motor 10 isdriven in rotation for rotating the rollers 63a to 63i at equal speedmatched to the supply rate of the strip A. In this manner, the strip Ais supplied sequentially to points m4, m3, m2 and m1 on the outerperiphery of the roller assembly 63 shown in FIG. 6. After one round ofthe strip A is wound around the outer periphery of the roller assembly63, that portion of the strip A which has made a complete round ispartially overlapped with a portion of the strip A subsequently suppliedto make the round in a similar manner, and is advanced by virtue of theinclination of the rollers 63a to 63i to form a complete turn of thehelix. The strip A is moved past the pressure roller 19 in the vicinityof the roller 63i so that the strip A is shaped in a corrugated shape bythe cross-sectional shape of the roller assembly in cooperation with thecontour of the pressure roller 19.

On the other hand, as the strip A is moved as described above, when thestrip A is supplied to both the convexed rings 62 and the follower rings61a provided adjacent to the convexed rings of each of the rollers 63ato 63i, the strip portions at the follower rings 61a are rotated at thefastest circumferential speed at each apex of the convexed rings 62without regard to the rotation at the axial center of each of therollers 63a to 63i. Since the follower rings 61a are rotated at a speedcorresponding to the circumferential speed at the apexes of the convexedrings 62 due to transfer of rotational force of the strip A withoutregard to the rotation of the rollers 63a to 63i, it becomes possible toeliminate the internal strain of the strip A due to the difference inthe circumferential speed between the apex of the convexed ring 62 andthe roll substrate 64. The result that the longitudinal creases usuallyproduced between the convexed rings 62 and the valley portions with theuse of conventional rollers not provided with the follower rings may beprevented from being produced. As a consequence, a spiral-shapedcorrugated tube having excellent durability may be produced by employingthe rollers 63a to 63i provided with the follower rings 61a.

In FIG. 9, 90 is a roller in which the pitches P₁ to P₆, diameters Q₁ toQ₇ and widths R₁ to R₇ of the convexed rings 92 are gradually reduced,in a manner similar to the rollers 3a to 3i shown in FIG. 2. Thefollower ring 91, like the follower ring 61a which is rotatably mountedvia dry bearing 61b on a positioning fixture 61c shown in FIG. 7, ismounted in the valley portions between the convexed rings 92

By employing the roller 90 in place of the rollers in the apparatus 1,60 shown in FIGS.1 or 6, is prevented the strip of synthetic resin isprevented from being detached from the roller surface due to contractionof the strip. Thus, it is possible to produce a corrugated tube which isexcellent in molding shape stability, accuracy and durability, and whichhas increased impact strength or pressure resistance. The tube is notsusceptible to internal stresses manifested as longitudinal creasesbetween the convexed rings and the valley portions between the convexedrings.

What is claimed is:
 1. An apparatus for producing a corrugated tubecomprising a roller assembly composed of a plurality of rollers, eachbeing rotatable at equal speed, said roller assembly substantiallyforming a single rotating axial body so that said rollers are arrayed ona cylindrical surface of an imaginary cylinder, roller supporting meansmounted axially opposite said roller assembly for rotatably supportingsaid rollers, means for continuously supplying a synthetic resin stripto an outer circumferential surface of said roller assembly from alateral side thereof, and driving means for rotating said rollers atequal speed, each axis of said rollers being inclined in acircumferential direction of the roller assembly, plural convexed ringsprovided along the axial direction of each of said rollers being arrayedso that the convexed rings form a helix in their entirety, with a pitchbetween the convexed rings and diameters and widths of said convexedrings on each of said rollers being gradually reduced in accordance witha contraction percentage of said synthetic resin strip at least within asection of said rollers in which the synthetic resin strip spirallywound and advanced along the axis of said roller assembly is contracted.2. The apparatus of claim 1 further comprising a pressure roller havingplural convexed rings mating with the convexed rings of at least one ofsaid rollers for fixing a shape of said strip wound and advanced in ahelix.
 3. The apparatus of claim 1 further comprising inner cylindricalwall forming means for forming a smooth cylindrical wall of syntheticresin on an inner peripheral surface of said imaginary cylinder forproviding a dual wall for the corrugated tube.
 4. The apparatus of claim1, wherein each axis of the rollers is further inclined toward a centralaxis of said roller assembly at an equal angle so that a diameter ofsaid imaginary cylinder is gradually decreased along the axiallyextending direction.
 5. The apparatus of claim 4 wherein each axis ofsaid rollers is further inclined toward the central axis of said rollerassembly by means of inclined angle adjustment means provided in atleast one of the roller supporting means.
 6. The apparatus of claim 1,wherein a rate of reduction of the pitch between the convexed rings andthe diameter and the width of the convexed rings is set so that0.8<Y/X<1, where X denotes each value of the pitch between the convexedrings and the diameter and the width of the convexed rings at asupplying point of said strip and Y is a minimum value thereof.
 7. Anapparatus for producing a corrugated tube comprising a roller assemblycomposed of a plurality of rollers, each being rotatable at equal speed,said roller assembly substantially forming a single rotating axial bodyso that said rollers are arrayed on a cylindrical surface of animaginary cylinder, roller supporting means mounted axially oppositesaid roller assembly for rotatably supporting said rollers, means forcontinuously supplying a synthetic resin strip to an outercircumferential surface of said roller assembly from a lateral sidethereof, and driving means for rotating said rollers at equal speed,each axis of said rollers being inclined in a circumferential directionof the roller assembly, plural convexed rings provided along the axialdirection of each of said rollers so that the convexed rings form ahelix in their entirety, with a pitch between the convexed rings anddiameters and widths of said convexed rings on each of said rollersbeing gradually reduced in accordance with a contraction percentage ofsaid synthetic resin strip at least within a section of said rollers inwhich the synthetic resin strip spirally wound and advanced along theaxis of said roller assembly is contracted, a follower ring beingprovided at each recessed valley portion between neighboring convexedrings, said follower rings being rotatable responsive to circumferentialspeed at an apex of said convexed rings.
 8. The apparatus of claims 7further comprising a pressure roller having plural convexed rings matingwith the convexed rings of at least one of said rollers for fixing ashape of said strip wound and advanced in a helix.
 9. The apparatus ofclaim 7 further comprising inner cylindrical wall forming means forforming a smooth cylindrical wall of synthetic resin on an innerperipheral surface of said imaginary cylinder for providing a dual wallfor the corrugated tube.
 10. The apparatus of claim 7, wherein each axisof the rollers is further inclined toward a central axis of said rollerassembly at an equal angle so that a diameter of said imaginary cylinderis gradually decreased along the axially extending direction.
 11. Theapparatus of claim 10 wherein each axis of said rollers is furtherinclined toward the central axis of said roller assembly by means ofinclined angle adjustment means provided in at least one of the rollersupporting means.
 12. The apparatus of claim 7, wherein a rate ofreduction of the pitch between the convexed rings and the diameter andthe width of the convexed rings is set so that 0.8<Y/X<1, where Xdenotes each value of the pitch between the convexed rings and thediameter and the width of the convexed rings at a supplying point ofsaid strip and Y is a minimum value thereof.